Wood has long been the material of choice for products such as crown molding, baseboards, door casings, and molding of all kinds. However, as the timber supply dwindles and concerns for the environment mount, wood has become more and more expensive both to procure and to machine into molding and other parts. In addition, the general quality of timber has declined over the years such that some high grades of lumber common 50 years ago are now simply not available.
Various attempts have been made to produce an extruded plastic material that can be used as a replacement for wooden building construction articles such as crown molding. Most of these attempts have resulted in an extruded cellular thermoplastic product of relatively low quality. While such products have been used in low end applications, they nevertheless tend to be plagued with various inherent problems and shortcomings. For example, in order to be economically competitive with wood, these thermoplastic products must be made with a relatively low density to minimize the amount of plastic in the material while maximizing the volume of air. The problem is that, when reduced to these densities, the extruded material lacks the structural constitution to hold nails and to be cut and worked like wood. Thus, the material is often glued to a wall and various contrivances are sometimes used to join pieces together at comers.
Another problem with prior art extruded moldings is that of providing a surface that either resembles wood or that is suitable for receiving and holding paint. A common prior art method of providing such a surface has been simply to cool the surface side of the extrusion as it exits the extruder. This prevents the blowing agent from expanding in the region of the surface and creates a relatively more dense and harder surface. However, this surface still tends to be somewhat robbery and does not hold nails or take paint well.
Polystyrene cellular materials are well known in the art. Polystyrene foams are used as coffee cups, packaging for food and industrial products, and as insulation board. Polystyrene foam, however, is brittle and would not serve as a wood replacement in and of itself due to its inability to be nailed, sawed, and generally manipulated without breaking. Rubber-modified styrene polymers, however, are more impact resistant. Such copolymers, called high-impact polystyrenes (HIPS), are sometimes used when toughness, flexibility, and resiliency are desired. Oftentimes, a mixture of HIPS and general purpose, or crystalline, polystyrene (GPPS) is used so that a product will have specific desired properties.
A method that has been applied to change the properties of GPPS products to produce a more suitable wood substitute is by utilizing multilayer structures. For example, U.S. Pat. No. 5,128,196 teaches a thermoplastic multi-layer sheet useful for packaging applications which comprises at least two layers, one of which is a thermoplastic foam layer and one which is a multi-resin layer comprising a mixture of at least three resins, one of which is an oxygen barrier resin. Multi-layer sheets are typically made by a coextrusion process requiting that the various layers will bond to each other. While this method is an improvement, it nevertheless is expensive and specialized and not generally suited to the production of wood substitute.
The prior art does not teach a foam polystyrene material useful as crown molding and the like. The compositions which have been taught do not provide adequate stiffness combined with non-brittleness so they can be handled and will not fracture when a nail is driven through while mounting. In addition, prior art products do not provide a surface which can be dyed or painted to a desired color.
Recycled polystyrene commonly is inconsistent in composition. Typically, the major component is GPPS. However, the recycled material also may comprise HIPS and other copolymers, dyes, grease, and other contaminants. The separation of pre-recycled material into various classes on the basis of composition is time and expense prohibitive to many recycled polystyrene providers. Therefore, the incorporation of recycled polystyrene into a finished product requires a certain amount of trial and error to arrive at consistent production results. However, the use of recycled polystyrene in constructive ways is of vital importance to our environment and may also be economically beneficial.
Foamed, also known as cellular, thermoplastic sheets are typically made by extrusion using a blowing agent, whereby polystyrene resin, for example, is heated in an extruder and is mixed in the extruder with a blowing agent and then extruded through a die and allowed to expand to form a polystyrene foam sheet. Common blowing agents are p,p'-oxybis (benzenesulfonyl hydrazide), azodicarbamide, alkaline earth metal carbonates or bicarbonates such as calcium carbonate, magnesium carbonate or sodium bicarbonate, and combinations of an alkaline earth metal carbonate or bicarbonate and one or more organic acids such as citric acid. As mentioned above, however, foamed thermoplastic alone is not suitable as a material for producing a substitute for wood molding or the like.
Thus, there is a need for an economically practical wood substitute appropriate for use as crown molding and the like that is stiff enough to hold its shape, that is not so brittle that it shatters upon impact, and that is provided with a surface suitable for coloring just as is a wood surface. There is also wide-spread need for providing constructive uses for recycled materials and for recycled polystyrene in particular. In addressing both of these needs simultaneously, the present invention provides a coextruded thermoplastic component comprised at least partially of recycled materials, suitable for use as a substitute for wood molding and having a constitution sufficient to be nailed and an outer surface suitable for painting.